1. Field of the Invention
This invention relates in general to magnetic sensors, and more particularly to a method and apparatus for enhancing thermal stability, improving biasing and reducing damage from electrostatic discharge in self-pinned abutted junction heads.
2. Description of Related Art
Magnetic recording is a key and invaluable segment of the information-processing industry. While the basic principles are one hundred years old for early tape devices, and over forty years old for magnetic hard disk drives, an influx of technical innovations continues to extend the storage capacity and performance of magnetic recording products. For hard disk drives, the areal density or density of written data bits on the magnetic medium has increased by a factor of more than two million since the first disk drive was applied to data storage. Areal density continues to grow due to improvements in magnet recording heads, media, drive electronics, and mechanics.
The use of a magnetoresistive (MR) and giant magnetoresistive (GMR) sensors to sense magnetically recorded data has been known for many years. The GMR sensor includes a spin-valve film that provides a high magnetoresistance ratio (MR ratio) as compared with a conventional MR head. The MR ratio is the percentage change in resistance as an external magnetic field is switched between high and low values.
It has also been known that due to spin-orbit coupling, some ferromagnetic materials used in forming a sensor display anisotropic resistivity, i.e. the resistivity being a function of an orientation between a current and a magnetic field. Hence, both traverse bias (i.e., the bias field is perpendicular to a preferred magnetization axis (easy axis—EA) and current direction) and longitudinal bias (i.e., the bias field is along the easy axis and current direction) of an MR structure must be provided to eliminate noise, such as Barkhausen noise, and to maintain the sensor in its most linear operating range.
In the search for new materials that will allow MR heads to be scaled to very small dimensions for increased areal densities and that will also maintain good signal amplitude, scientists have developed films, which exhibit the GMR effect. GMR heads are made up of four layers of thin material that combine into a single structure. A free layer is the sensing layer. The free layer is passed over the surface of the data bits to be read. It is free to rotate in response to the magnetic patterns on the disk. A pinned layer is a layer that is held in a fixed magnetic orientation by its proximity to an exchange layer. The exchange layer is a layer of antiferromagnetic material that fixes the pinned layer's magnetic orientation. A spacer, typically made from copper, is a nonmagnetic layer that separates the magnetization of the free and pinned layers. When the head passes over a magnetic field of one polarity, the electrons on the free layer turn to align with those on the pinned layer, creating a lower resistance in the head structure. When the head passes over a field of opposite polarity, the free layer electrons rotate so that they are not aligned with the electrons on the pinned layer. This causes an increase in the structure's resistance.
To avoid noise, a longitudinal bias field along the current direction of the free layer element is needed. There are two popular longitudinal bias schemes for GMR heads: exchange bias and hard bias. Exchange bias refers to the unidirectional pinning of a ferromagnetic layer by an adjacent antiferromagnet. Ferromagnetic films typically have a preferred magnetization axis, easy axis, and the spin direction preferably aligns along this axis. Hence, there are two equally stable easy spin directions (rotated by 180°) along this axis and it requires the same energy and the same external field to align the spins along either direction.
The ferromagnetic layer may be magnetically pinned or oriented in the fixed and unchanging direction by an adjacent anti-ferromagnetic layer (AFM), commonly referred to as the pinning layer, which pins the magnetic orientation of the ferromagnetic layer (i.e., the pinned layer) through anti-ferromagnetic exchange coupling by the application of a sense current field. Also, the ferromagnetic layer may be self-pinned, in which the magnetic moment of the pinned layer is pinned in a fabrication process, i.e.—the magnetic moment is set by the specific thickness and composition of the film. The self-pinned layer may be formed of a single layer of a single material or may be a composite layer structure of multiple materials. It is noteworthy that a self-pinned spin valve requires no additional external layers applied adjacent thereto to maintain a desired magnetic orientation and, therefore, is considered to be an improvement over the anti-ferromagnetically pinned layer.
In a hard bias scheme, such as abutted junction hard bias, two hard magnets abut at least the free layer along a longitudinal direction. The hard (bias) magnets include a hard magnetic layer such as CoPtCr and appropriate under-layer and/or overlayer for desirable magnetic and electrical properties. The hard magnets are electrically connected to the free layer allowing sense current (IS) to pass through. A magnetostatic field generated by the hard magnets serves the longitudinal bias field (HI).
However, the longitudinal schemes discussed above are sensitive to electrostatic discharge (ESD) and high temperatures produced thereby. Electrostatic discharge can be manifest on giant magneto resistive (GMR) head either physically (e.g., melting of a sensor) or magnetically (e.g., degrading electrical characteristics of a sensor).
Damage to a sensor form ESD occurs in different levels. If the energy level is high enough, ESD will burn and/or melt GMR stripes resulting in bumps on an air-bearing surface (ABS). For example, high ESD energy absorbed at the two ends of an abutted junction of a MR sensor may form bumps, including fractured stripes between the bumps, on the sensor. Also, high ESD energy absorbed along stripes of GMR sensor can melt the sensor forming several micro metal balls. A low ESD energy level may only damage the magnetic structure of an MR sensor (usually call as “soft” ESD).
In addition, the central active area between MR leads and hard bias layers tends to operate at high temperatures. Hence, the added energy from ESD can physically damage the sensor and/or cause unpinning of a pinned layer.
An alternative longitudinal bias scheme is a lead-overlay (LOL) (i.e., overlay hard bias scheme), in which the leads define a trackwidth of a written track. A self-pinned LOL is less sensitive to ESD than the self-pinned abutted junction head, however the LOL head does not provide a desired narrow trackwidth, e.g., below 0.15 um. Therefore, the self-pinned abutted junction head is preferable from an areal density perspective.
It can be seen then that there is a need for a method and apparatus for enhancing thermal stability, improving biasing and reducing damage from electrostatic discharge in self-pinned abutted junction heads.